This invention relates to a heat-resistant alumina fiber mat and, more particularly, to a heat-resistant alumina fiber mat suitably employed as a fire-resistant and heat-insulating material provided on the inner sides of industrial heating furnaces, such as those used in iron and steel making, in the field of ceramics or in chemical plants. The present invention also relates to a process for producing such a heat-resistant alumina fiber mat.
At the present time, mats in the form of blankets formed of amorphous ceramic fibers containing 40 to 60 wt. % of Al.sub.2 O.sub.3 and 60 to 40 wt. % of SiO.sub.2 and blocks produced therefrom, are used extensively. Since the maximum temperature of these ceramic fiber mats to be used is 1200.degree. C., crystalline alumina fibers containing 70 to 95 wt. % of Al.sub.2 O.sub.3 and 5 to 30 wt. % of SiO.sub.2 are recently evolved for application in industrial heating furnaces operated at still higher temperatures.
However, since the crystalline alumina fibers are lower in strength than the amorphous ceramic fibers, the fibers tend to be crushed to pieces when they are subjected to needle punching employed in ceramic fibers, so that it is difficult to produce the mats of higher strength. For this reason, the crystalline alumina fiber mats are produced by the wet process comprising firstly pulperizing the alumina fibers in water, followed by addition of a suitable binder and felting. However, since the fibers are cut by this method to lengths of several millimeters during the pulperizing operation, the fibers may be entangled only to a limited extent. Moreover, after an organic sizing agent has burnt off at higher temperatures, the tensile strength is lowered to 50 to 100 g/cm.sup.2, while the peeling strength is also as low as 5 g/cm.sup.2 at most. The term `tensile strength` herein means the strength along the length or width of the mat formed of fibers, while the term `peeling strength` means the strength along its thickness.
On the other hand, it has been attempted to use the fibers in the state of fiber lengths obtained directly after accumulation. For example, there is proposed in the Japanese Laid-open Patent Publication No.110439/1985 a process for producing an alumina fiber mat containing an organic non-woven fabric as the reinforcement by needle punching. However, this method is inconvenient in that the alumina fibers are crushed to pieces during needle punching, and in that the organic non-woven fabric is burnt off when the mat is used at elevated temperatures, while the peeling strength cannot be elevated because of decrease in the mat density.
There is further proposed in the Japanese Laid-open Patent Publication No. 252717/1985 a process for producing the mats in the form of blankets comprising needle punching an accumulated mass of a precursor of alumina fibers, that is, an intermediate product of alumina fibers obtained before sintering, and sintering the obtained precursor. With this process, it is presumably possible to produce mats with higher strength than that of those produced by the conventional processes. However, by this process, the peeling strength has an extremely low value of not more than 1 g/cm.sup.2. Also, for producing the mats of higher tensile strength, the accumulated body of the precursor may be effectively humidified and pressurized for providing higher mat density without using expensive needle punching machines. However, in this case, only an extremely small value of the peeling strength has similarly been obtained.
As described hereinabove, it has been difficult with the prior art to produce a highly heat-resistant alumina fiber mat having high peeling strength in addition to high tensile strength at elevated temperatures proper to the highly heat-resistant crystalline alumina fibers.